1. Field of the Invention
The present invention relates generally to circuits and methods for reducing the signal peaks in telecommunications signals, and in particular to circuits and methods for reducing the peak to average power ratio of non-constant envelope modulation signals. The present invention is suitable for use in applications involving multi-code Code Division Multiple Access (CDMA) signals, Orthogonal Frequency Division Multiplexing (OFDM) signals and critically sampled complex Gaussian signals, and it will be convenient to described the invention in relation to these exemplary applications. It will be appreciated, however, that the invention is not limited to use in these applications only.
2. Description of Related Art
Telecommunications devices that transmit signals with a high peak to average power ratio (PAPR), such as multi-code CDMA signals, require output power amplifiers with large peak power capabilities. Such amplifiers are large and costly, and have high power requirements. If such amplifiers cannot handle the peak power signal levels then the resulting saturation causes inter-modulation products and Adjacent Channel Interference (ACI). A number of techniques exist for reducing the peak to average ratio, but many of these are modulation dependent, for example, coding, partial transmit sequences, or phasing for (OFDM) and Multi-Carrier Spread Spectrum.
FIG. 1 illustrates a portion 1 of the transmission path of a multi-code CDMA transceiver, in which a signal's in-phase and quadrature (I & Q) components are up-converted by an up-converter 2 or a number of upconversion stages for amplification by an amplifier 3, and subsequent transmission. A pulse-shaping interpolation filter 4 or other filtering is used to band limit the signal supplied to the up-converter 2. One effective method of preventing saturation of the amplifier 3 is to introduce a signal peak clipper before the amplifier 3. There are two possible positions in a transmitter for the clipper, namely the placement of a clipper 5 before the filter 4 or the placement of a clipper 6 after the filter 4. Placing a clipper after the filter prevents amplifier saturation, eliminating AM to PM distortion in the amplifier saturation region, but still generates ACI. The latter can be reduced by peak windowing, or eliminated all together by putting the clipper before the filter. However, the placement of the clipper 5 before the filter 4 can cause regrowth of the clipped peaks, which can again cause amplifier saturation. Harder clipping, or over-clipping, can prevent peak re-growth but attenuates the signal more than necessary.